1. Field of the Invention
The present invention relates to a method of treating the surface of an absorbent resin. More particularly, it relates to a method of treating the surface of the absorbent resin by crosslinking the surface region of the absorbent resin homogeneously and effectively using a crosslinking agent to obtain an absorbent which is excellent in absorption rate under pressure, water-retaining property under pressure and liquid permeability, and is suitable for use as a material in a sanitary article such as a sanitary napkin, a disposable diaper, etc.; as a water-retaining agent for agriculture and horticulture and aforestation; and as a material for other various absorbent articles.
2. Description of the Prior Art
Attempts have been made heretofore to use an absorbent resin as one of the component materials for such sanitary articles as sanitary napkins and disposable diapers which function to absorb body fluids. Absorbent resins of this nature heretofore known to the art include a hydrolyzed starch-acrylonitrile graft polymer (Japanese Patent Publication SHO 49(1974)-43,395), a neutralized starch-acrylic acid graft polymer (Japanese Patent Laid-Open SHO 51(1976)-125,468), a saponified vinyl acetate-acrylic ester copolymer (Japanese Patent Laid-Open SHO 52(1977)-14,689), a hydrolyzed acrylonitrile copolymer or acrylamide copolymer (Japanese Patent Publication SHO 53(1978)-15,959), cross-linked products thereof, a partially neutralized polyacrylic acid, and a partially neutralized cross-linked polyacrylic acid (Japanese Patent Laid-Open SHO 57(1982)-34,101).
Characteristic properties expected in absorbent resins include high absorption capacity, outstanding absorption rate and liquid permeability, and large gel strength. These characteristic properties, however, are not effected simultaneously. That is, there are disadvantages that an absorbent resin having a high gel strength possesses a low absorption capacity, the absorbent resin having a high absorption capacity possesses a low absorption rate because of gel blocking phenomenon or low gel strength after absorption. Attempts have been made to increase the absorption rate by decreasing the particle size of the absorbent resin, granulating the absorbent resin, or forming the absorbent resin in flakes. Generally, when the absorbent resin is formed in a small particle size, the resin particles on contact with urine convert themselves into what resembles wetted clusters of flour possibly to an extent of lowering the absorption rate. When the absorbent resin is formed in the form of granules, there ensues a phenomenon that the granules themselves independently convert into wetted clusters on contact with urine and the absorption rate is lowered. When the absorbent resin is formed in the form of flakes, though the absorption rate is fairly improved because the gel blocking does not occur, the absorption rate is not sufficient and the production of the absorbent has a restriction from the standpoint of process because the formation of the absorbent resin in the form of flakes is not economical because the produced absorbent resin inevitably becomes bulky and necessitates larger facilities for transportation and storage.
On the other hand, a technique for improving the absorption rate and the gel strength after absorption is by crosslinking molecule chain on the surface region of the absorbent resin to increase the crosslinking degree of the surface region without substantially decreasing the absorption capacity.
These techniques have been disclosed in Japanese Patent Laid-Open SHO 57(1982)-44,627, Japanese Patent Laid-Open SHO 58(1983)-42,602, Japanese Patent Publication SHO 60(1985)-18,690, Japanese Patent Laid-Open SHO 58(1983)-180,233, Japanese Patent Laid-Open SHO 59(1984)-62,665 and Japanese Patent Laid-Open SHO 61(1986)-16,903, for example. The absorption rate and gel strength after absorption can be improved by the above mentioned surface treating techniques, but there are some disadvantages in that it requires a large amount of an organic solvent or is insufficient in mixing when the surface treating agent is mixed with the absorbent resin.
On the other hand, recently, it has been found that the water retaining property and absorption rate under pressure are required as properties of the absorbent resin. Further, when the absorbent resin is used for a disposable diaper, the absorbent resin is used by dispersing among the pulp fibers, and liquid high permeability is required in such a case.
Accordingly, an object of the present invention is to provide a novel method of treating the surface of an absorbent resin which is industrially and economically excellent.
An object of the present invention is to provide a method of treating the surface of the absorbent resin which is effective for obtaining an absorbent possessing high absorption rate under pressure and high water retaining property under pressure.
Still another object of the present invention is to provide a method of treating the surface of the absorbent resin which is effective for obtaining the absorbent possesing excellent liquid permeability when it is used by dispersing among pulp fibers and the like.